The present invention relates to the production of slab products made of a granulated stone material or sand bound with a hardenable resin and, more specifically, to an improvement in the production line thereof.
The method of producing these slabs to which the present invention relates is the method according to which a mixture constituted by granulated material of a selected size and by synthetic resin is deposited in metered quantities on a conveyor belt which is advanced to a forming station (in which the mixture is subjected to a compaction operation under vacuum with simultaneous application of a vibration of predetermined frequency) and subsequently to a station in which the resin is hardened (preferably by the action of a catalyst and/or heat). Upstream of the forming station, the upper surface of the metered quantity of mixture deposited on the conveyor belt is covered by a sheet or layer of protective material which prevents the pressure plate of the press from being soiled by the mixture.
In the past, this protective sheet material was paper, the use of which, however, was accompanied by some problems collateral to and downstream of the actual production line but none the less of considerable importance.
In fact, after the hardening of the resin, the paper sheet remained strongly adhering to the surface of the hardened slab and was quite difficult to remove therefrom so that a suitable operation for the mechanical removal of the protective paper was required during the finishing of the slab.
Subsequently, an improvement (described and claimed in European published Patent Application No. 0 786 325) was introduced, according to which the paper layer or sheet was replaced by a sheet of resilient material preferably rubber.
Upon completion of the hardening of the binding resin, the rubber sheet can thus in fact be pulled away from the suds of the finished slab and subsequently reused.
In the preferred embodiment, the support on which the mixture is deposited before being transferred to the station for compaction under vacuum and with vibration is also protected by a similar sheet of resilient material, particularly rubber.
After the forming stage, the mixture deposited on the support and enclosed between the two rubber sheets is in the form of a slab at the edges of which the two protective rubber sheets are joined together by their respective overlapping edges, substantially completely enclosing the raw slab which has been formed but not yet subjected to the step of hardening of the resin binder.
Since, during compaction, a portion of the mixture, albeit a minimal portion, inevitably forms a flash interposed between the two edges, this flash forms deposits on the rubber sheets after the resin binder has been hardened and these deposits are difficult to remove because the two rubber edges do not remain in contact with one another during the hardening step, so that the as yet unhardened flash is detached from the body of the product.
To prevent this problem, again in the previously known method, a release liquid is applied, immediately before use on the production line, to the two edges which are intended to meet. In spite of these measures, however, operations to clean the two rubber sheets, particularly in the region of the meeting edges are still necessary, although these operations are certainly less onerous than those previously connected with the removal of protective paper, they nevertheless necessitate a further operation in the production cycle in addition to that of the application of the release liquid to the two edges of the rubber sheets.
Moreover, in order to meet the peripheral edge of the lower sheet in the region of the edges of the formed raw mixture and of the adjacent side thereof the peripheral edge of the upper rubber sheet has to be deformed, overcoming the natural resilience which would tend to return it to the completely flat configuration.
The main object of the present invention is to solve these problems in an industrially advantageous manner.
A more specific object of the present invention is to improve the method and plant described above so as to avoid the need for the application of release liquid and at the same time to prevent the formation of deposits on the rubber sheets, which necessitate laborious cleaning operations.
These objects are achieved by the present invention by modifications both to the sheets and to the method of operation.
In the first place, the flat configuration of the lower sheet is replaced by a shaped configuration comprising a flat base and a peripheral frame projecting from the flat base for a predetermined height so as to define a seat of dimensions corresponding in plan to those of the slab to be produced, but the aforementioned peripheral frame is given a height which is lower by a predetermined amount than that of the final slab to be produced so that, after the compaction step, a space of predetermined depth remains between the peripheral edge of the upper sheet and the top of the aforementioned frame.
Excess mixture material is disposed in this space of predetermined depth during the step of vibratory compaction under vacuum and this material then remains attached to the edge of the slab during the hardening stage. When the final slab is pulled free of the two rubber sheets after the hardening of the resin binder, the hardened raw slab obtained has a peripheral frame of hardened material which can easily be removed during normal finishing operations for a slab of stony material.
As will become clearer from the detailed description with reference to the drawings, with the present invention, when the upper sheet is pulled away from the now hardened slab, the material forming the slab is easily separated from the rubber sheet since the force applied by the sheet to the surface of the slab is tangential so that, even at the edges of the slab, the rubber sheet is separated in the same manner and with the same results (that is, without the material of the slab remaining adhering to the adjacent surface of the rubber sheet) as were achieved for the central portion of the rubber sheet in the known method.
With reference row more specifically to the shaped configuration of the lower sheet, that is, the sheet holding the mixture initially deposited, its peripheral rim or, more precisely, the inner surface thereof which restrains the mixture material may adopt various configurations which are based on an analysis of the forces acting on the inner surface of the rim and at the same time on the consequences with regard to waste material at the edge of the slab.
In other words, the inner surface of the rim or frame of the lower sheet may adopt a shape or orientation which varies from a substantially vertical shape (in which case, the lower sheet is shaped like a box open at the top and with side walls perpendicular to the base surface) to a shape inclined in various ways (in which the lower sheet again forms a kind of box having an edge or fame with a flared arrangement).
Now, the force acting on the inner surface of the rim is approximately perpendicular thereto and, for inclined surfaces in general, can be broken down into two components, that is, a horizontal component and a vertical component, respectively. The horizontal component is that which causes deformation of the resilient rim (which results in a force opposing the detachment of the rubber from the adjacent surface of the slab).
On the other hand, the greater the outward inclination of the inner surface, the more easily the slab cart be extracted.
With regard to case of extraction of the slab, and in order to create a force component which squeezes the rubber rim towards the press plate, an inner surface of the rim of the lower sheet inclined outwardly as much as possible (basically with an external angle of between 90xc2x0 and 0xc2x0) therefore seems preferable.
However, a greater inclination of the inner surface of the rim of the lower sheet corresponds to a greater width of the corresponding strip of waste material, whereas, if the peripheral rim is perpendicular to the base surface of the sheet, the waste material is reduced to the maximum possible extent, but the aforesaid horizontal component increases to the maximum extent. The inner surface of the frame or rim of the lower sheet should therefore be shaped in a manner such as to increase the vertical component or force to the maximum possible extent and at the same time to reduce the amount of waste material on the edge of the hardened slab to the maximum possible extent. This situation generally corresponds to a surface inclined at an angle of less than 90xc2x0 and preferably of the order of 45xc2x0.
An upwardly concave curved shape of the inner or restraining surface of the rim or frame is also possible; in this case, most of the force is in fact discharged vertically onto the base rather than laterally.